1. Field of the Invention
The present invention relates to a semiconductor laser device provided with an external resonator including a wavelength selection element. The present invention also relates to a semiconductor laser module including a semiconductor laser device and an optical wavelength conversion element which converts the wavelength of a laser beam emitted from the semiconductor laser device, to generate its second harmonic or the like.
2. Description of the Related Art
As disclosed in Japanese Unexamined Patent Publication No. 10(1998)-254001, conventionally, laser light generated by a semiconductor laser device is converted into its second harmonic by using a wavelength conversion element. That is, the wavelength of the laser light emitted from the semiconductor laser device is reduced by half. In addition, an attempt has been made to use a second harmonic having a blue or green wavelength for recording a color image in an optical-scan type recording apparatus such as a laser printer.
On the other hand, as disclosed in Japanese Unexamined Patent Publication No. 10(1998)-254001, another attempt has been made to lock the wavelength of laser light emitted from a semiconductor laser device at a desired wavelength by combining an external resonator with the semiconductor laser device when laser light emitted from the semiconductor laser device is converted into its second harmonic, where the external resonator contains a wavelength selection element such as a narrow-band-pass filter.
In the case where a semiconductor laser device is coupled to an external resonator including a wavelength selection element, if the passband of the wavelength selection element is broader than the Fabry-Perot mode interval (i.e., the difference between wavelengths of two adjacent Fabry-Perot (FP) modes, among a plurality of FP modes), which are determined by the interval of a pair of cleavage planes of the semiconductor laser device, laser oscillation in the semiconductor laser device occurs in a plurality of longitudinal modes. For example, when the Fabry-Perot mode interval in the semiconductor laser device is about 0.2 nm, and the width of a passband of the wavelength selection element is 0.5 nm, laser oscillation in the semiconductor laser device may occur in two or three longitudinal modes, where the width of the passband of the wavelength selection element is the half-value width of the passband, i.e., a bandwidth in which the transmittance of the wavelength selection element is equal to or more than half of the maximum value of the transmittance in the passband.
In the above situation in which the laser oscillation in the semiconductor laser device occurs in a plurality of longitudinal modes, the allocation of power to the respective longitudinal modes may vary with time, even if the driving current of the semiconductor laser device is fixed. This phenomenon occurs when a specific amount of driving current is supplied to the semiconductor laser device, and is called longitudinal mode competition.
In FIGS. 3A and 3B, the transmittance of the narrow-band-pass filter is indicated by curves, and the light intensities of the three longitudinal modes are indicated by vertical bars. As illustrated in FIGS. 3A and 3B, the light intensities of the respective longitudinal modes vary due to the variation in the allocation of power to the respective longitudinal modes.
When the longitudinal mode competition occurs, the intensity of light which is fed back to the semiconductor laser device varies with time (i.e., with the varying oscillation state of the external resonator), due to the difference in the transmittance of the wavelength conversion element at the wavelengths of the respective longitudinal modes. Accordingly, the oscillation state of the semiconductor laser device also varies with time, and therefore the intensity of the output of the semiconductor laser device varies with time. When the output light of the semiconductor laser device is converted into the second harmonic, the second harmonic also varies with time.
In addition, in the case where the second harmonic is generated by using a wavelength conversion element, for example, the tolerance of the wavelength for phase matching is about 0.2 nm when the coupling length is 6 mm, and the wavelength of the second harmonic is in the range of 450 to 550 nm. That is, the tolerance of the wavelength for phase matching is smaller than the bandwidth of the oscillation wavelength of the semiconductor laser device. Therefore, when the substantial oscillation wavelength varies due to the longitudinal mode competition, the efficiency of the wavelength conversion also varies. Thus, the variation of the luminous energy of the second harmonic is further increased due to the small tolerance of the wavelength for phase matching.
The frequency of the variation of the luminous energy of the second harmonic caused as above is in the range of 0 to 10 MHz. Therefore, when the above second harmonic is used for recording a color image in an optical-scan-type recording apparatus such as a laser printer, the variation of the luminous energy of the second harmonic causes noise or unevenness in the recorded image.
An object of the present invention is to provide a semiconductor laser device comprised of a semiconductor laser element and an external resonator including a wavelength selection element, wherein occurrence of longitudinal mode competition can be prevented even when the passband of the wavelength selection element includes a plurality of wavelengths of a plurality of Fabry-Perot modes of the semiconductor laser element.
Another object of the present invention is to provide a semiconductor laser module comprised of a semiconductor laser element, an external resonator including a wavelength selection element, and an optical wavelength conversion element performing wavelength conversion on laser light emitted from the semiconductor laser element, wherein intensity variation in the wavelength-converted laser light can be suppressed even when the passband of the wavelength selection element includes a plurality of wavelengths of a plurality of Fabry-Perot modes of the semiconductor laser element.
According to the first aspect of the present invention, there is provided a semiconductor laser device which includes a semiconductor laser element, an external resonator which is coupled to the semiconductor laser element, and a high-frequency superimposing unit.
The semiconductor laser element has a pair of cleavage planes, and laser oscillation in a plurality of Fabry-Perot modes can be realized between the pair of cleavage planes. The external resonator includes a wavelength selection element having a passband which includes more than one wavelength of more than one Fabry-Perot mode out of the above plurality of Fabry-Perot modes. The high-frequency superimposing unit superimposes a high-frequency current on a driving current of the semiconductor laser element.
According to the second aspect of the present invention, there is provided a semiconductor laser module which comprises the above semiconductor laser device according to the first aspect of the present invention, and an optical wavelength conversion element which converts the wavelength of the laser light emitted from the semiconductor laser device.
Due to the superimposition of the high-frequency current on the driving current of the semiconductor laser element, the driving current of the semiconductor laser element does not stay in a region in which the longitudinal mode competition occurs. That is, due to the superimposition of the high-frequency current, the driving current quickly passes through the above region. Therefore, the longitudinal mode competition can be suppressed, and the variation in the luminous energy of the second harmonic can be suppressed.
In the first and second aspects of the present invention, it is preferable that the degree of modulation of the driving current with the high-frequency current is in the range of 70% to 100%. It is further preferable that the degree of modulation with the high-frequency current is 100%.
In the case where the wavelength-converted laser light which is output from the semiconductor laser module according to the second aspect of the present invention is used as recording light in the optical-scan-type recording apparatus, it is preferable that the frequency of the high-frequency current is greater than a reciprocal of a cycle time of an operation of writing a pixel in an optical-scan-type recording apparatus, and smaller than a response frequency of the semiconductor laser device. When the frequency of the high-frequency current is within this range, visible noise in an image printed by the optical-scan-type recording apparatus can be suppressed.
In addition, when the wavelength-converted laser light which is output from the semiconductor laser module according to the second aspect of the present invention is used as recording light in the optical-scan-type recording apparatus, it is preferable that the frequency of the high-frequency current is in the range of 10 MHz to 2 GHz. It is further preferable that the frequency of the high-frequency current is in the range of 10 MHz to 50 MHz.